Process for separating microscopic algae

Abstract

Microscopic algae, for example of the Spirulina or Oscillatoria species, are isolated from aqueous suspensions thereof according to a multi-step process which comprises a pre-concentration step in which the suspension is fed along a filtration surface at a high contact velocity, followed with a filtration, a washing and a pressing out step. Filtering devices of the drum type, the continuous belt type or the inclined plane type are also disclosed and claimed.

Description

The invention relates to a process for separating microscopic algae.

It has for object to provide a process for separating microscopic algae from suspensions thereof, particularly from the aqueous nutritive media in which they have grown.

It has for particular object to provide a process for separating microscopic algae from diluted suspensions thereof.

The separation process of the invention applies particularly to the diluted suspensions of algae having a filamentous or helical shape, such as, for example, the algae of the Spirulina or Oscillatoria species.

It is known that such microscopic algae as the Cyanophytae, for example Spirulina and Oscillatoria, are a source of proteins and thus are highly interesting as food for humans and animals.

These microscopic algae, which are obtained in the form of relatively diluted suspensions in the natural or synthetic nutritive liquid media in which they have grown, cannot be used as food, except if made free of the said media and the inorganic or organic elements that they retain, and dried.

These suspensions usually have an algal concentration of 0.01 to 3 g (dry weight) per liter.

According to their species, these microscopic algae have various shapes and sizes, from a few microns to a few hundreds of microns. For example, the Spirulina alga has the shape of an helix of about 200-400 microns length, consisting of turns of about 20 microns diameter.

The separation of the microscopic algae from the nutritive solutions in which they have grown sets a difficult problem which has not, up to now, received a satisfactory solution.

Attempts have been made to separate them by settling, but this technique cannot be applied easily since the low density difference between the liquid phase and the algal phase requires very long operations, which are sometimes disturbed by the floating resulting from the oxygen produced by the algae cells in the medium.

With regard to the centrifugation techniques, they are expensive and have the further inconvenience of breaking a great part of the algae cells; it results therefrom a substantial loss of the content of said cells, and consequently a loss of proteins.

Therefore, the filtration and/or pressing-out techniques appear as being the most suitable ones. However, they cannot easily be operated on account of the size of the cells to be recovered, their mechanical strength, the high amount of liquid to be separated and the nature of the latter, which is strongly clogging since it has a high content of organic materials.

Thus, the conventional filtration devices, for example the vacuum drum filters and the belt filters are useless for the filtration of such suspensions, mainly since the filtration cake formed on the filtration cloth very quickly clogs the meshes of this cloth which forbids the satisfactory continuing of the operation.

It has now been found that the separation of microscopic algae by filtration may be carried out provided this operation mainly comprises two filtration steps: the first one consists in filtering the algae suspension under such specific conditions, as to avoid the clogging of the fabric and to obtain a concentrated suspension which has not the same defects as the initial diluted suspension with respect to the conventional filtration techniques; the second one is a conventional filtration steps.

The separation process of this invention comprises a first pre-concentration step, in which a suspension of microscopic algae is fed onto a filtration surface in such manner that the suspension, when contacting said filtration surface, has a sufficiently high relative contact velocity to carry away the forming filtration cake in the form of a concentrate suspension and a second step, in which the said concentrated suspension is filtered through a filtration surface, thus retaining an algae cake. It further comprises a third step, in which the said cake is washed by feeding water therethrough, a fourth step, in which water is pressed out from the washed cake, a fifth step, in which the resulting cake is converted to a fluid mud by mechanical breaking of a fraction of the algae contained therein, and a sixth step, in which the said fluid mud is transferred to a drying zone where it is dried. Each of the filtration surfaces used in this process has mesh size of 10-100 microns. These surfaces may particularly consist of fabrics made of synthetic gut such as Rilsan (trade mark) gut. Nylon, polyester or stainless steel gauges may also be used.

In the first step of the process, the suspension to be pre-concentrated has a velocity, when the kinematic energy thereof contacts the filtration surface, that a part of the liquid content thereof, usually 0.5-20% and preferably 0.5-5% by volume, does not filter through the filtration surface but flows therealong, thus carrying away the filtration cake which tends to form, so as to constitute a concentrated suspension which may thereafter be filtered under conventional conditions in the second step.

According to the volume of the liquid fraction carried away by the filtration surface, the initial suspension is usually concentrated by 5 to 200 times, preferably 20 to 200 times, in this pre-concentration step.

The resulting concentrated suspension usually has an algal concentration from 1 to 30 grams (dry weight) per liter, preferably from 1 to 10 grams per liter.

In the second step of the process, the concentrated suspension recovered from the first step is filtered under conventional conditions.

The third step consists in withdrawing the salts which contaminate the cake by water-washing, for example by means of one or more pulverization pipes and under sucking of the water through the filtration surface. This water-washing is sufficient to limit the ash-content to a value lower than the maximum tolerated by the standards for food products (i.e. 5% by weight).

In the fourth step, water is pressed out from the washed cake, for example to a water-content from 80 to 83% by weight, for example by sucking thereof through the filtration surface.

The resulting cake appears as a plastic solid material which cannot be pumped.

It must be subjected to a treatment consisting in a fifth step, in a partial mechanical breaking of the algae cells contained therein. This mechanical breaking, which permits the fluidization of the said cake, may be carried out according to various techniques, for example by means of a mixer, a grinding pump or an emulsion homogeneizer: in the latter apparatus, the cake is subjected to a quick pressure release, from a pressure higher than 100 bars down to the atmospheric pressure.

In the sixth step, the fluidized cake is dried according to a conventional technique, for example by atomization or on rolls.

The first step preconcentration operation according to the invention may be carried out with a number of filtration devices which are conceived in such manner that the algae suspension to be pre-concentrated, when contacting the filtration surface, is moved at a relative velocity sufficient for impeding the formation of a cake sticking on the said filtration surface.

Four particular devices which may be used to carry out this pre-concentration step, are described hereinafter with reference to the FIGS. 1, 2, 2A, 3, 3A, 4, 4A, 4B, and 4C. FIG. 5 shows a perferred association of filtration devices.

A first device consists of an inclined plane filter (FIG. 1), which mainly comprises a plane filtration cloth 1 inclined with respect to an horizontal plane and whose inclination angle (.phi.) may range, for example, from 2.degree. to 85.degree..

In this device, the inclination of the filtration cloth (1) by a selected angle value (.phi.), usually from 10.degree. to 60.degree., preferably from 10.degree. to 30.degree., permits the flowing of the suspension along the said filtration cloth, so as to impede the formation of an adherent filtration cake.

The filtration cloth 1, usually of rectangular shape, may be placed on a plane carrier, itself rectangular, consisting of a grid, and maintained on the latter, for example by means of a fixed frame 6.

The device also comprises a feed box 2 along the higher edge of the filtration cloth, a collector 3 along the lower edge of the said cloth for receiving the concentrated suspension resulting from the filtration and a recovery tank 4 below the filtration cloth 1 for collecting the filtrates.

The device may also comprise a multiple position supporting device and a hinge along the lower edge of the carrier 5 of the cloth 1, so that the said carrier 5 may revolve to take any of the positions provided on the carrier 7.

The suspension to be pre-concentrated is fed from a pipe 9 into the feed box 2 which may be provided with baffle-plates such as 10 for reducing the turbulence caused in the feed box 2 by the supply thereinto of the suspension from the pipe 9, so that a smooth flow of the suspension through the slot 11 of the said box 2 is obtained.

The suspension then flows along the filtration cloth 1 as a uniform sheet. The concentrated algae suspension which forms on the cloth 1 is carried along by the unfiltered liquid fraction down to the lower edge 1 of the said cloth, wherefrom it is collected in collector 3, from which it may be withdrawn by pumping through pipe 12 to be fed to the filtration zone of the following step. The filtrate is collected in the recovery tank 4, from which it may be flowed through pipe 13.

The diluted suspension to be pre-concentrated may be fed at an hourly rate of 10-50 m.sup.3 per sq.m. of filtration area. The concentration of the resulting suspension may be, for example, 1-30 grams (dry weight) per liter. It is usually 5-15 g per liter.

A second pre-concentration device consists of a vertical drum rotative filter, as shown on FIGS. 2 and 2A, which mainly comprises a vertical cylindrical drum able to revolve around its hollow shaft and whose external surface of the vertical wall, consisting for example of a grid or a perforated wall, supports the filtration cloth 17.

This drum 14 is placed in a fixed tank 18 itself cylindrical and of same axis as the drum 14 and whose vertical wall is provided with at least one baffle-plate such as 19, consisting, for example, of a rectangular plate attached to the said vertical wall of the tank 18.

This device also comprises a feeding pipe 20, placed above the annular zone 21 located between the vertical wall 16 of the drum 14 and the vertical wall of the tank 18, and a stationary set of pipes, placed inside the drum 14, for the withdrawing of the filtrates. Said set of pipes comprises a vertical pipe 22 leaving the drum 14 through its hollow shaft 15 and receiving the extremity of at least one afferent pipe as 23, the other extremity of which is located near the vertical wall 16 of the drum 14.

The device also comprises a pipe 24 for withdrawing the concentrated mud from the tank 18.

The device is fed with the algae suspension to be pre-concentrated from pipe 20. The drum 14 revolves around its axis, for example at such a speed as to give to its vertical wall 16 a linear velocity of 0.5 - 5 meters per second.

The filtration of a selected fraction of the liquid volume through the filtration cloth 17 is obtained by pumping through the pipes 22 and 23.

In this device, the baffle-plate 19, by disturbing the flowing of the suspension in the annular zone 21, impedes the said suspension to be circulated at the same speed as the drum 14. The difference in speed between the filtration cloth and the suspension to be pre-concentrated (relative contact velocity along the said cloth) impedes the formation and/or the sticking of a filtration cake on the cloth 17. The diluted suspension to be pre-concentrated may be fed at an hourly rate of 3 - 10m.sup.3 sq.m. of filtration surface.

The concentrated suspension obtained in this operation has, for example, an algal concentration of 3 - 8 g (dry weight) per liter.

According to another embodiment, the device has the same features as above, except that the vertical axis of the drum 14 does not coincide with the vertical axis of the tank 18 and the vertical wall of the tank has no baffle-plate such as 19. The flowing of the suspension to be pre-concentrated around the drum is no more disturbed by such baffle-plates as 19 but by the fact that the horizontal cross-section of the annular zone 21 has not a constant width all around the drum 14, and the narrowing (resp. the widening) of this cross-section resulting in an acceleration (resp. a slowing down) of the suspension flow.

A third device which may be used in the pre-concentration step of the process of the invention consists of a fixed drum-filter as shown on FIGS. 3 and 3A, which mainly comprises a drum 25 of vertical axis in the shape of a truncated cone enlarging towards the top and whose internal surface of the inclined wall 26, consisting for example of a grid or a perforated wall, supports the filtration cloth 27. The drum 25 is placed in a stationary tank 28 for example having the shape of a truncated cone and the same axis as the drum 25. The device also comprises at least one feed pipe such as 29, which opens along the higher side of the drum 25 wall, a pipe 30 for withdrawing the filtrates and a pipe 31 for withdrawing the concentrated suspension.

The suspension to be pre-concentrated is injected onto the filtration cloth from pipe 29 at a contact speed sufficient for impeding the formation and/or the sticking of an algae cake. The injected suspension turns inside the drum 25 as a vortex. The concentrated suspension accumulate at the bottom of the drum 25, wherefrom they are withdrawn through pipe 31. The filtrate passing into tank 28 is withdrawn through pipe 30.

A fourth pre-concentration device consists of a belt filter as shown on FIGS. 4, 4A and 4C. It comprises a continuous mobile cloth belt 32 which is moved along a path at least of part of which is horizontal. This device also comprises at least one feed device 33 on FIG. 4 for pouring the suspension on the horizontal portion of the cloth 32 at a contact velocity, in the same direction as the cloth is moved, which is sufficient for impeding the formation or the sticking of a filtration cake on the said cloth 32.

The feed device 33 may consists, as shown for example on FIG. 4A, of a grazing injection pipe fed from a pipe under pressure 35. Said injection pipe 34 may consist of a pipe provided with a longitudinal slit 36. The plane comprising the slit 36 axis and the pipe 34 axis and the plane of the filtration cloth 32 meet at an angle .alpha. of 10.degree. - 45.degree. and preferably 20.degree.-40.degree..

The device for feeding the algae suspension to be pre-concentrated may also consist of a feed box under atmospheric pressure, such as shown by the respective reference 37 and 40 on FIG. 4B and 4C. The height of said feed-box must be sufficient for feeding the suspension flowed therefrom through slit 38 or over-flow shoot 41 at the required contact velocity. The axial plane of the slit 38 or of the over-flow-shoot 41 meets the plane of the filtration cloth 32 at an angle of 10.degree. - 45.degree. and preferably 20.degree. - 40.degree. (.beta. on FIG. 4B and .alpha. on FIG. 4C). The feed boxes 37 and 40 may be provided with either horizontal 39 on FIG. 4B or vertical 42 on FIG. 4C baffle-plates, so as to reduce the turbulence of the suspension contained therein.

Several feed devices such as 33 may be provided along the horizontal path of the moving cloth FIG. 4.

The cloth speed is usually from 0.5 to 20 centimeters per second. The suspension to be pre-concentrated is injected on the cloth 32 at an incidence angle of 10.degree. - 45.degree. and preferably 20.degree. - 40.degree. and, for example, at a velocity of 1 - 10 meters per second.

A concentrated suspension is collected at the end of the horizontal portion of the filter. The device may treat an amount of suspension corresponding, for example, to 3 - 20 kg of dry material per sq. meter of cloth and per hour.

Although the devices used for the first pre-concentration step of the process are so conceived as to impede the formation of a clogging cake, it is necessary for each of them that the filtration cloth be subjected to a washing step. The washing is intermittent and usually requires the stopping of the filtration step, regarding the fixed or mobile drum filters. Conversely, regarding the inclined plane filter or the belt filter, it may be carried out during the filtration step.

In each case, the washing step consists of passing pressurized water through the filtration cloth, usually in the opposite direction with respect to the filtrates, or in the same direction, regarding the inclined plane filter.

The second, third and fourth steps of the process may advantageously be carried out by using different zones of a single device, for example, a rotative filter or a belt filter.

Usually, the device consists of a mobile continuous cloth belt which receives the concentrated suspension from the first step. The cloth first goes through a filtration zone where the suspension is usually subjected to a pressure decrease of less than 100 grams per sq. cm. preferably 4 - 25 grams per sq.cm., so as to form a filtration cake whose thickness is usually a few centimeters, for example 4 - 20 cm. The cake-bearing cloth is then transferred to a washing zone in which water is passed through the cake subjected to a pressure decrease which is not limited as before and may attain 750 g/sq.cm. The cloth bearing the washed cake then passes through a pressing-out zone, where said cake is subjected through the cloth to a pressure decrease which may also attain about 750 g/sq.cm. The resulting cake is then discharged from the cloth by means of a knife or a pneumatic system.

The cloth made free of the cake is then transferred into a washing zone, where pressurized water is passed therethrough, usually in opposite direction with respect to the filtrates. The washed cloth is finally returned to the initial filtration zone to receive a new charge of the algae suspension to be treated. With the said device (rotative filter or belt filter) the filtration, washing and pressing-out steps may be carried out continuously.

When constructing the whole separation plant as defined by the invention any device of the first (pre-concentration) step as hereinbefore described may be associated with any device of the second, third and fourth steps (filtration, washing, pressing-out) also hereinbefore described.

A preferred association comprises the use of the same belt filter for the pre-concentration step and the three following steps. Such a device is shown on FIG. 5.

In that case, the same filtration cloth belt 32 comes along a pre-concentration zone 43, where it is fed from 33 as described with respect to the pre-concentration step, a filtration zone 44, a washing zone 45 and a pressing-out zone 46. The cake is discharged at 50 and the cloth goes into the washing zone 51. The sucking tanks such as 47 and 48 produce the pressure decrease required in the filtration, washing and pressing-out zones.

Injection nozzles such as 49 and 52 are respectively used for washing the cake in the washing zone 45 and washing the cloth in the washing zone 51.

Claims

What we claim is:

1. A process for separating microscopic algae from a dilute aqueous suspension thereof, comprising a first step, in which the aqueous suspension of microscopic algae having an algae content of about 0.01 to 3 grams of dry weight per liter is fed onto a filtration surface, with such a relative velocity that said suspension, when contacting the filtration surface, carries away any forming filtration cake, and that a portion of about 0.5 to 20% of the liquid volume of the said suspension does not filter through said filtration surface resulting in a concentrated suspension, a second step, in which the said concentrated suspension is filtered through a filtration surface which retains an algae cake, a third step, in which the said cake is washed by feeding water therethrough, a fourth step, in which water is pressed out from the washed cake, a fifth step, in which the resulting cake is converted to a fluid mud by mechanical breaking of a fraction of the algae contained therein and a sixth step, in which the said fluid mud is transferred to a drying zone where it is dried, the filtration surfaces having each a mesh size in the range of 10-100 microns.

2. A process according to claim 1, wherein the microscopic algae are of the Spirulina or Oscillatoria species.

3. A process according to claim 1, wherein the relative velocity of the suspension, when contacting the filtration surface in the first step, is such that 0.5 - 5% of the liquid volume of the said suspension flows along the filtration surface.

4. A process according to claim 1, wherein the concentrated suspension issuing from the first step has an algal concentration of 1 - 30 grams (dry weight) per liter.

5. A process according to claim 1, wherein the concentrated suspension issuing from the first step is filtered under a pressure decrease lower than 100 grams per sq. cm., in the second step.

6. A process according to claim 1, wherein the filtration cake issuing from the second step is washed with pressurized water, under sucking of water through the filtration surface, in the third step.

7. A process according to claim 1, wherein water is pressed out from the washed cake issuing from the third step, by sucking through the filtration surface, in the fourth step.

8. A process according to claim 1, wherein the cake issuing from the fourth step is passed through a mixer, a grinding pump or an emulsion homogeneizer in the fifth step.

9. A process according to claim 1, wherein the fluidized cake issuing from the fifth step is dried by atomization or by passage on rolls, in the sixth step.

10. A process according to claim 1, further comprising a step of washing the filtration surface after the first step by means of pressurized water.

11. A process according to claim 1, wherein the filtration of the second step, the washing of the third step and the pressing-out of the fourth step are carried out on the same filtration surface which is successively passed through three different zones and the pressed-out cake is discharged at the end of the fourth step.

12. A process according to claim 11, wherein the second, third and fourth steps are carried out on a rotative filter.

13. A process according to claim 11, wherein the second, third and fourth steps are carried out on a belt filter.

14. A process according to claim 11, wherein the filtration surface is passed through a washing zone where it is washed by pressurized water flowing in the reverse direction with respect to the filtrates, after discharge of the pressed-out cake.

15. A process according to claim 1, wherein, in the first step, the suspension is fed along the higher edge of a filtration cloth, substantially plane and inclined with respect to an horizontal plane, by such an angle that said suspension is provided with a velocity sufficient to carry away any forming filtration cake in the form of a concentrated suspension which is carried along said filtration cloth by the unfiltered part of the said suspension and is received at the lower edge of said filtration cloth.

16. A process according to claim 15 wherein the suspension is fed at an hourly rate of 10-50 m.sup.3 per square meter of filtration area of said filtration cloth, inclined with respect to an horizontal plane by such an angle that 0.5 to 20% of the liquid volume of said suspension flows along said filtration cloth.

17. A process according to claim 15, wherein said substantially plane filtration cloth is inclined with respect to an horizontal plane by an angle of 10.degree.-60.degree..

18. A process according to claim 15, wherein said substantially plane filtration cloth is inclined with respect to an horizontal plane by an angle of 10.degree.-30.degree.C.

19. A process according to claim 1, wherein, in the first step, there is used a rotative drum filter comprising a rotative, vertical, cylindrical drum, the vertical perforated wall of which supports a filtration cloth on its external surface, said drum being placed in a stationary cylindrical tank of same axis as said drum, the vertical wall of said tank being provided with at least one baffle-plate on its internal surface, and wherein the suspension, fed to the annular zone located between the external vertical wall of said drum and the internal vertical wall of said tank, is circulated in said annular zone by the rotation of said drum, the circulation of said suspension being disturbed by said baffle-plate in such a manner that, during filtration, by pumping through said filtration cloth, of a fraction of the liquid volume of said suspension, the relative velocity of said suspension in contact with said filtration cloth is sufficient to carry away from the surface of said filtration cloth any forming filtration cake, in the form of a concentrated suspension, which is received at the bottom of said annular zone.

20. A process according to claim 19, wherein the suspension is fed at an hourly rate of 3-10 m.sup.3 per square meter of filtration area of the filtration cloth and the drum revolves around its axis at such a speed as to give its vertical wall a linear velocity of 0.5-5 meters per second.

21. A process according to claim 1, wherein, in the first step, there is used a rotative drum filter comprising a rotative, vertical, cyclindrical drum, the vertical perforated wall of which supports a filtration cloth on its external surface, said drum being placed in a stationary cylindrical tank the axis of which is distinct from the axis of said drum, and wherein the suspension, fed to the annular zone located between the external vertical wall of said drum, and the internal vertical wall of said tank is circulated in said annular zone by the rotation of said drum, the circulation of said suspension being disturbed by the variation of the cross-section of said annular zone all around said drum, in such a manner that during filtration, by pumping through said filtration cloth, of a fraction of the liquid volume of said suspension, the relative velocity of said suspension in contact with said filtration cloth is sufficient to carry away from the surface of said filtration cloth any forming filtration cake in the form of a concentration suspension, which is received at the bottom of said annular zone.

22. A process according to claim 21, wherein the suspension is fed at an hourly rate of 3-10m.sup.3 per square meter of filtration area of the filtration cloth and the drum revolves around its axis at such a speed as to give its vertical wall a linear velocity of 0.5-5 meters per second.

23. A process according to claim 1, wherein in the first step, there is used a stationary drum filter comprising a stationary drum of vertical axis in the shape of a truncated cone enlarging towards the top, the inclined perforated wall of which supports a filtration cloth on its internal surface, said drum being placed in a stationary tank, and wherein the suspension is fed along the higher edge of the internal surface of said drum, in such a manner that the suspension circulates inside said drum at a speed sufficient to carry away from the surface of said filtration cloth any forming filtration cake, in the form of a concentrated suspension which is received at the bottom of said drum.

24. A process according to claim 1, wherein in the first step, there is used a belt filter comprising a mobile, continuous perforated belt supporting a filtration cloth and moved along a path, at least a portion of which is included in a horizontal plane, and wherein the suspension is fed on a horizontal portion of said filtration cloth in the same direction as said filtration cloth is moved and at such a speed and under such an incidence angle that the velocity of said suspension in contact with said filtration cloth is sufficient to carry away any forming filtration cake, in the form of a concentrated suspension, which is subsequently carried by the movement of said filtration cloth along said horizontal portion of the path and is discharged at the end of said horizontal portion of the path.

25. A process according to claim 24, wherein the suspension is fed on a horizontal portion of the filtration cloth, moved at a speed of 0.5 to 20 cm/s, under an incidence angle of 10.degree.-45.degree. and at a speed of 1-10 m/s.

26. A process according to claim 25, wherein said incidence angle is 20.degree.-40.degree..

27. A process for separating microscopic algae from a dilute aqueous solution having an algae content of about 0.01 to 3 grams of dry weight per liter comprising the steps of feeding said aqueous suspension onto a filtration surface with such a velocity relative to the filtration surface that said suspension, when contacting the filtration surface, carries away any forming filtration cakes, and that a portion of about 0.5 to 20% of the liquid volume of said suspension does not filter through said filtration surface, resulting in a concentrated suspension, and filtering said concentrated suspension to form a filtration cake of algae on a filtration surface.

28. A process for separating microscopic algae from a dilute aqueous suspension thereof comprising a first step in which the aqueous suspension, having an algae content of about 0.01 to 3 grams of dry weight per liter, is fed onto a filtration surface with such a relative velocity that said suspension, when contacting the filtration surface carries away any forming filtration cake and that a portion of about 0.5 to 20% of the liquid volume of said suspension does not filter through said filtration surface, resulting in a concentrated suspension, a second step in which said concentrated suspension is filtered through a filtration surface which retains an algae cake, a third step, in which the said cake is washed by feeding water therethrough, a fourth step, in which water is pressed out from the washed cake, a fifth step, in which the resulting cake is converted to a fluid mud by mechanical breaking of a fraction of the algae contained therein and a sixth step, in which the said fluid mud is transferred to a drying zone where it is dried, the filtration surfaces having each a mesh size in the range of 10-100 microns, and wherein the concentration of the first step, the filtration of the second step, the washing of the third step and the pressing-out of the fourth step are carried out on the same filtration surface of a belt filter, which is successively passed through four different zones, and the pressed-out cake is discharged at the end of the fourth step, and wherein said belt filter comprises a mobile, continuous perforated belt supporting a filtration cloth and moved along a path, at least a portion of which is included in a horizontal plane, and wherein the suspension is fed on a horizontal portion of said filtration cloth in the same direction as said filtration cloth is moved and at such a speed and under such an incidence angle that the velocity of said suspension in contact with said filtration cloth is sufficient to carry away any forming filtration cake, in the form of a concentrated suspension, which is subsequently carried by the movement of said filtration cloth along said horizontal portion of the path and is discharged at the end of said horizontal portion of the path.